Fidelity control circuits



Feb. 18, 1936. v DOWDING v2,03l034 FIDELITY CONTROL CIRCUITS Filed Nov. 16, 1935 y /7 fw? INVENToR GEORGE .DOWDING ATTORNEY Patented F eb. 18, 1936 ETE ST TES ATET OFFICE FIDELITY CONTROL CIRCUITS poration of Delaware Application November 16, 1933, Serial No. 698,311 In Great Britain November 16, 1932 7 Claims.

The present invention relates to improvements in wireless receiving circuits, and is more particularly concerned with receiving circuits employing tone control arrangements.

It is Well known that in a selective wireless receiver employing reaction whereby the degree of selectivity may be regulated the passage of a modulated wave through the receiver is accompanied by a higher attenuation of the high frequency components than of the low frequency components. This high-note loss is compensated for by the employment of an amplifier stage designed to accentuate the high notes more than the low notes. This method of tone correction, however, does not take into account the variation in the amount of reaction or degree of selectivity employed when receiving different stations. For instance, when receiving signals from a local station normally no reaction is employed and, consequently, the high note loss is than if signals were being received from a distant station. High note compensation will, however, take place to the same extent as if signals were being received from a distant station and distortion will consequently take place.

It is the object of the invention to obviate the above mentioned disadvantage and, to this end, provision is made of a circuit such that when receiving signals from a local station, when normally no reaction is employed, that is, when the circuit is broadly tuned and is not highly selective, the high note loss is increased to bring it to the same level as when signals are received from a distant station. Tone correction is then applied and distortion will not take place.

In the accompanying illustrative drawing:-

Fig. 1 is a circuit diagram of one embodiment suitable for illustrating the purpose and method of operation of the invention:

Figs. 2 to 4 are circuit diagrams of three other embodiments of the invention.

Referring to Fig. 1, I indicates a thermionic valve arranged as a grid-leak rectifier and having an oscillatory input circuit II, I2 fed by any known means and an output circuit comprising a high-frequency choke I3 in series with the primary I4 of a low-frequency coupling transformer forming part of a subsequent amplifier I5; the latter (or the transformer) is designed in known manner to give greater amplification to the higher frequencies than to the lower frequencies fed thereto and hereinafter will be referred to as the tone correcting amplifier. This amplifier, then, has a rising amplification characteristic.

Reaiim feed-959k t9 the input Crut iS Pro- (Cl. Z50-20) vided by means of a reaction coil I6 in series with an adjustable or pre-set condenser I1 connected between the anode and the cathode (or earth line) of the valve III, the control of reaction or selectivity of the input circuitbeing effected by means of a second shunt path for the high-frequency output componentcomprising a variable condenser I8 in series with the usual high-frequency by-pass condenser I9 also connected between the anode and cathode of the valve I 0. The condenser I8 is provided with means as indicated at 20 for the short-circuiting thereof at its maximum capacity setting.

In operation, during the reception of a strong signal such as that emanating from a local station, when normally no reaction will be applied to the input circuit II, I2, which will then be broadly tuned, the condenser I8 will be set at maximum capacity in order to provide a low impedance shunt, and thus prevent high frequency energy from passing around the reaction coil I 6. When in this position the condenser I8 is short-circuited at contacts 20, and as a result the by-pass condenser I9 is placed directly across the input transformer of the tone correcting amplifier I5, thus to achieve a high-note loss, which by suitable choice of capacity of the condenser I9 can be made to equal the amount of high-note gain subsequently effected by the tone correcting amplifier I5.

When reaction is applied to the input circuit II, I2 such as is necessary for the reception of a weak signal, that is, when the input circuit is sharply tuned or highly selective, considerable high-note loss is caused, but by reason of the adjustment of the condenser I8 to effect a decrease in the capacity thereof in order to obtain the said reaction, the short circuit by way of contacts- 20 is removed from around the condenser I 8. The by-pass condenser I9 is thus rendered less effective and the high-note gain of the amplifier I5 acts to compensate for the losses involved by the use of reaction.

As the resultant capacity of the condensers I8 and I9 in series, and thus also the high-note loss occasioned thereby, decreases in accordance with the amount of reaction applied to the input circuit II, I2, or with its degree of selectivity, it is obvious that the high-note gain of the amplifier I5 becomes more and more effective to compensate for the high-note loss occasioned by the increased use of reaction.

In the modified arrangement of the circuit shown in Fig. 2, the thermionic valve Ill has an oscillatory input circuit I I, I2 and output circuit lll comprising choke I3 and transformer primary I4 as previously described with reference to Fig. 1. The reaction coil I5 coupled to the input circuit II, I2 has its ends connected, one to each of the xed ele-ments ZIE, 2lb of a differential condenser 2I. The moving element ZIc of the said condenser is connected to the cathode (or earth line) of the valve III.

The differential condenser 2I is so arranged that when set in one maximum position, i. e. with the entire moving element 2|c in register with the fixed element 2 Ia and clear of the other Xed element 2 Ib, the moving element 2 Ic is electrically connected with the opposing fixed element 2IEL by way of contacts 22. A xed condenser 23 is connected between the anode of the valve IIJ and the Xed element 2 Iab of the condenser 2|.

In the operation of this embodiment of the invention, during the reception of a strong signal Without the use of reaction the condenser 2l is set with the moving element 2Ic in register with the fixed element 2iEL and electrical connection is made between them by way of the contacts 22.. This has the eiect of isolating the reaction coil I5 and at the same time places the condenser 23 as a shunt` directly across the input transformer of the amplifier I5; thus, as previously described with reference to Fig. l, a high-note loss is effected for subsequent correction by the characteristics of the amplier I5.

When the differential reaction condenser ZI is adjusted to apply reaction to the input circuit I I, I2 the moving element 2 Ic is so placed that it is .at least partly in registerwith the xed element 2 I b and some high-frequency energy is fed to the input circuit II, I2 by way of the condenser 23, coil I8 and condenser elements 2lb,

The connection between the condenser elements 2I1=- and 2lc having been broken, the high-note `loss caused by the condenser 23 is reduced and the high-note gain of the amplier I5 acts to compensate for the loss occasioned by the increased use of reaction.

The two parts of the diierential condenser may be of unequal capacity for the purpose of obtaining a graduated degree of high-note loss.

In the embodiment shown in Fig. 3 the reaction-control condenser I'I is mechanically coupled (ganged) to a variable condenser 25 shunted across the transformer I4, the condenser 2li taking the place of the condensers I 8, I9 in Fig. 1. In Fig. 4 a similar system is shown, the only difference from Fig. 3 being that the condenser shunt is replaced by a shunt path comprising a variable resistance 25 ganged to the condenser Il and a xed condenser 26.

Although the thermionic valve I0 is shown in the drawing as a leaky-grid detector provided with capacity-controlled reaction and transformer coupling to the succeeding amplifier, it is obvious to those skilled in the art that various modifications involving other methods of rectification, reaction and coupling arrangements may be made without departing from the scope of the invention, as defined in the appended claims. Moreover, the invention may be applied at a diierent stage of a wireless set.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:

1. In a radio receiver, an electron discharge tube provided with a tunable signal input circuit, an output circuit connected to the output electrodes of said tube, a network following said tube adapted to amplify the modulation frequencies of received signals, said network having a rising frequency-amplification characteristic,

means operatively associated with said tube for regulating the selectivity characteristic of said signal input circuit, and additional means, operative upon adjustment of said first means to impart minimum selectivity to said tuned input circuit when a strong signal is received, for diverting from the input to said network currents of the higher modulation frequencies.

2. In a system as dened in claim 1, said electron discharge tube having included in its input circuit means for rendering the tube operative as a signal detector.

3. In a system as defined in claim 1, said rst means including an adjustable signal frequency coupling path between the input and output circuits of said tube.

4. In a system as defined in claim 1, said additional means including an attenuation path connected to attenuate .thehigher modulation frequencies, said path being connected across the input of said network.

5. In a radio receiver, an electron discharge tube provided with a tunable signal input circuit, an output circuit connected to the output electrodes of lsaid tube, means connected to the input circuit of said tube for rendering the tube operative as a signal detector, an audio amplier network coupled to the output circuit of said detector tube, said ynetwork having a characteristie such that it has a rising modulation fre- CII quency amplification characteristic, a signal coupling path connected between the input and output circuits of-said detector tube and including an adjustable condenser for regulating the degree of coupling, and a path connected across the input of said audio amplier network for transmitting ythe higher modulation frequencies,

the last path including an adjustable transmis-4 sion control element.

6. In a system as defined in claim 5, said transmission control element comprising a varii l GEORGE VICTOR DOWDING. 

